The present invention relates to a display apparatus, and more particularly, to a display apparatus of a so-called head-up display system in which predetermined image information is superposed on the scene ahead of the apparatus by means of a beam combiner which has a semitransparent mirror, a hologram and other components, so as to put the image to be observed in a field of view of the observer.
A display apparatus laving a beam combiner which has a hologram is disclosed, for example, in U.S. Pat. No. 4,218,111. This apparatus is mounted, for example, in front of a driver's seat of a vehicle such as an automobile, pilot's seat of an aircraft, and so forth, so as to enable the driver or the pilot to simultaneously observe both the scene and the image information.
FIG. 1 shows a system in which a display apparatus of the kind described is used on an automobile. The apparatus has a hologram 100 which functions as a beam combiner. The hologram 100 is formed on the glass of the front window of the automobile. In this Figure, hatched areas L and R represent, respectively, the regions which can be observed by the left and right eyes of the observer, i.e., the driver, when the hologram 100 is positioned 800 mm ahead of the center 51 of observation. Two elongated circles 51a and 51b show ellipses of the right and left eyes which represent statistical standard positions of the eyes. The ellipses 51a, 51b have sizes of 95 percentile in terms of the definition specified by JIS (Japanese Industrial Standards) D0021.
The ellipses 51a, 51b in FIG. 1 are illustrated on the assumption that the driver's seat is shiftable back and forth by 140 mm. The most preferred condition for display in an automobile is that while area of the display is covered by the regions L and R, it is observable by the left and right eyes corresponding to 95 percentile. It is to be understood, however, that only a limited region where two hatched areas L and R overlap each other is observable by both eyes simultaneously. It is assumed here that the hologram 100 on the front window glass has a width of 100 mm. In such a case, the width of the region 101 observable by two eyes at a plane A which is 100 mm ahead of the hologram 100 is 93 mm. Thus, the width of the region 101 is substantially the same as that of the hologram 100. Thus, when the image information is displayed on the plane A, the driver can observe the image information without moving his head, so that the eclipse of image is avoided even when the hologram has a width which is as small as 100 mm. Hitherto, however, it was impossible to form an image on the plane A, since the known holograms are constructed to serve as a concaved mirror for the purpose of enlarging the image to be displayed. When such a known hologram is used,the image is formed on a plane B which is 400 mm ahead of the front window glass. In this case, the width of the region which can be observed by both eyes is as small as 67 mm. This narrow region can contain only a limited quantity of image information, e.g., three characters or letters. The above-described problems are caused not only by a lateral shift of the observing position but also by a vertical shift of the same. Namely, the observing position, i.e., the positions of the eyes, varies according to the sitting height of the driver and angle of inclination of the seat back. Therefore, in the conventional hologram which has only a limited region of observation in the vertical direction, the driver is often obliged to straighten his upper body or to crouch, in order to observe the image information formed on the plane B.